The present invention relates to the spray application of liquid coatings to articles in a production line process. In particular, the invention concerns spray line apparatus and methods used to apply resin in the commercial production of multi-layer laminate products such as plywood.
Most plywood plants use what is known in the industry as a "spray line" to apply resin to and assemble layers of veneer to make plywood. A typical spray line is schematically represented in Prior Art FIG. 1. The spray line includes a continuous conveyor 1 with a number of "drop stations" 3 arranged therealong. Drop stations 3 are where successive layers of wood veneer 5 are placed on top of each other to build-up the layers of the panels. For simplicity, only four drop stations are shown in FIG. 1. For the production of plywood, a spray line will commonly include a total of ten drop stations, corresponding to the ten layers of two five layer plywood panels to be produced. Initial positioning of the veneer layers at the drop stations is done automatically by a conventional conveyor apparatus (not shown). A closer alignment of the panels is performed manually by an attendant, as necessary. Eight of the drop stations are arranged adjacent to, and directly upstream of, a spray booth 7 where resin is applied to the top surface of the veneer layer just "dropped." Once the resin is applied, another layer of veneer is added at the next station. This process is repeated until a complete plywood panel has been assembled. At the fifth drop station, the last layer of a first panel assembly formed in the line is applied. The build-up of a second panel assembly, on top of the first, is started at the sixth drop station, which is located adjacent the fifth station (with no spray booth in between). The second panel assembly is completed at the tenth drop station. Next, a stacked load of thirty to forty of the resultant panel-forming assemblies is pre-pressed as a batch. This is carried out at ambient temperature and a pressure of about 150 psi, in a conventional pre-press apparatus (not shown), to make the individual panel assemblies rigid enough to be hand loaded into a standard hot press. In the hot press (not shown), panels are hot-pressed individually between plates heated to 280.degree.-330.degree. F., under a pressure of about 175 psi.
Each spray booth includes an open-bottom box-shaped enclosure (omitted for clarity) in line with the conveyor, which houses a spraying apparatus. The conveyor carrying the veneer mat moves continuously through the open-ended bottoms of the spray booths. In each booth, resin is pumped under pressure through a downwardly directed spray nozzle (also referred to as a "wand") 9. Nozzles 9 produce a generally flat spray pattern 10 having an inverted triangular shape viewed in the moving direction of the conveyor. The spray pattern envelopes the entire width of the veneer mats as they move through the booth. The edges of the spray pattern extend beyond the edges of the veneers, creating an overspray 11. This overspray drains through a trough 13 at the bottom of the booth and is recovered for reuse with a conventional recycle circuit comprising a resin tank 15, pump 17 and pressure header 19.
Typically, resin flow rates and pressures range from 2 to 3 gallons per minute and 100 to 200 psi respectively. The amount of resin applied to the veneer (this is measured as a "spread rate," i.e., weight of resin applied per unit area of veneer surface) is critical. Depending on various process parameters, e.g., the thickness and moisture content of the veneer layers, veneer temperatures and the ambient temperature, the ideal flow rate may vary from 30 to 60 lb/mft.sup.2. Usually, a phenolic based resin with a resin solids content of around 30% is used.
The amount of resin applied to each layer of veneer at a given flow rate depends on the distance from the veneer surface to be coated to the spray nozzle, which is usually 30" to 60". The nozzle (which is movably mounted) is lowered closer to the veneer to increase the spread rate. It is raised to decrease the spread rate. The conventional technique for adjusting the spread rate is to manually raise or lower the nozzle on a trial and error basis until the desired spread rate is achieved. Spread rates are measured using a standard sized (e.g., 4".times.47") metal test strip (nominal thickness of about 1/16") that is placed on the veneer and passed through the spray pattern. The amount of resin (by weight) on the test strip is determined as the difference in the weight of the test strip before and after spraying; the spread rate is determined from the resin weight and a chart which converts weights to per-unit densities, based upon the top surface area of the test strip. Once the desired spread rate has been achieved, the nozzle is fixed at a height above the conveyor corresponding to the desired spread rate.
A shortcoming of the above-described conventional process/apparatus is that it does not take into account variations in process parameters (e.g., resin flow rates and line speeds) that may cause the actual spread rate to deviate significantly from the desired spread rate during a production run. In order to avoid the possibility of a reduction in the spread rate resulting in improper lamination, the target spread rate is typically set higher than would otherwise be required. This results in higher resin consumption and costs than would be necessary if a desired spread rate could be more reliably maintained. In addition, excess resin application can lead to defective lamination of the veneer layers. For example, during the hot pressing operation, excessive moisture resulting from an excessive amount of applied resin may vaporize and build-up pressure within the panels until a "blow" occurs causing separation of the veneer layers.
A related problem is the effect of variations in the temperature of the veneer on the set-up or thickening of the applied resin prior to pressing. At the time of pressing, a degree of curing of the resin layer to a tacky state is desirable; however, a proper lamination will not be formed if the resin has cured excessively prior to the hot-pressing operation. Such curing and thickening of the resin occurs more rapidly if the temperature of the veneer is elevated. In this case, application of the resin at a higher spread rate can compensate for the increased curing rate and ensure an appropriate degree of tackiness of the resin at the time of hot pressing. In the conventional technique, no provision other than ad hoc and occasional adjustment for observed/sensed temperature variations is made to account for such in-process variations in the temperature of the veneer.
One known spray control system, offered by Drying Technology Co. of Salsbee, Tex., sought to maintain an optimum spread rate by automatically varying the height of the spray nozzle above the conveyor in relation to detected changes in (1) a pressure of the resin supply line at the spray nozzle; (2) conveyor speed; and (3) a detected temperature of the veneer. The present inventor is unaware of the particular control algorithm of this system. In any event, he found that this system did not satisfactorily achieve its objective of consistently maintaining an optimum spread rate.